The present invention relates to a surface-treated polyester film. More particularly, the present invention relates to a coated polyester film having excellent antistatic property, adhesivity and solvent resistance, useful as a base material for highly processed polyester film products such as magnetic tape, floppy disc, X-ray photographic film, telephone card, OHP, base for printing plate and the like, as well as to a process for producing said film.
A biaxially stretched polyester film, particularly a biaxially stretched polyethylene terephthalate film has excellent mechanical properties, heat resistance, chemical resistance, etc. and is used as a base material for magnetic tape, photographic film, wrapping film, metallized film for capacitor, electrical insulating film, OHP film, etc. The recent growth in demand for said film is very striking. The polyester film, however, has drawbacks in that since the surface is highly crystallized and oriented, the film shows high surface cohesivity and poor adhesivity to various coatings.
Hence, in order to improve the surface adhesivity of the polyester film, it is known to employ various means, for example, a corona treatment, a plasma treatment and a flame treatment. These means, however, have a drawback in that the effect imparted thereby is reduced with the lapse of time.
As a means for eliminating the above drawback, a treatment by chemical agent is mentioned. This means, however, is disadvantageous in practical application because the chemical agent is toxic and the vapor generated therefrom causes environmental pollution.
As another means, it is known to coat a coating agent on a biaxially stretched polyester film to form a primer layer of good adhesivity on the film. However, this means also has safety and hygiene problems such as deterioration of environment by vaporization of the organic solvent used in the coating agent, and further has a fear that surface defects are frequently generated as a result of dust deposition because the coating environment is dirty.
Accordingly, if the formation of a primer layer on a polyester film is made during the step of producing the polyester film by using an aqueous coating agent, it is effected in a clean environment, the degree of dust deposition is low, there is neither fear of explosion nor deterioration of environment because there is formed an aqueous layer, and this approach is advantageous also in properties of resulting film, economy and safety.
Because of these advantages, it has been proposed to use a water-soluble or water-dispersible polyurethane as a primer.
For example, U.S. Pat. No. 4755337 discloses a process for producing a polyester film having a primer layer, which comprises coating an aqueous primer coating composition comprising a polyurethane having at least one of a carboxylic acid salt group, a sulfonic acid salt group and a hemisulfate salt group, and a polyepoxy compound on at least one surface of a polyester film before its crystallization and orientation are completed, and thereafter completing the crystallization and orientation by subjecting the coated film to drying, stretching and heat treatment.
A polyester film also has a drawback in that it is electrified easily. For example, the magnetic recording medium using, as its base, a primer-treated polyester film of good adhesivity as mentioned above, is electrified by the static electricity generated during tape running or disc rotation and invites dust deposition thereon and the resultant drop-out of recording and regeneration. Further, in winding-up, rewinding or slitting in the steps of film or magnetic recording medium production, there occur in some cases the emittance of spark from the film caused by peeling and electrification, the breakage of the film caused by electrification and bonding, and so forth. Therefore, particularly the film for magnetic recording media is required to have an antistatic property.
As the method to impart an antistatic property to a magnetic recording medium using a polyester film, there can be mentioned conventional methods such as (1) a method wherein a conductive powder such as carbon powder, metal powder or the like is dispersed uniformly in a magnetic layer, (2) a method wherein an antistatic agent such as surface active agent is compounded into a polyester film by kneading, (3) a method wherein a thin metal layer is formed on a film surface by physical vapor deposition (PVD), and (4) a combination of the above methods. These methods, however, have various problems. For example, the method (1) has problems such as reduction in rentention caused by incorporation of conductive particles into the magnetic layer, resultant reduction in recording density, acceleration of drop-out caused by detachment of conductive particles, and so forth. The method (2) has problems in that the antistatic agent incorporated by kneading does not bleed out easily onto the film surface, the antistatic property of the film surface decreases with the lapse of time, and the surface resistance increases correspondingly therewith. The method (3) has problems in that the adhesion between the film and thin metal layer formed thereon is weak and the thin metal layer is detached during use.